I think the answer to your question has several layers. I’ll start with the most general layer and get more specific from there…

The ActivityPub protocol, which Lemmy and Mastodon (and other services) use to communicate, is published by W3C and was developed by a group of people. This page on W3C includes a list of authors. It looks like at least one author has a Mastodon account; I’m curious to see if mentioning them here will federate to Mastodon and get a response: @[email protected]

The main developers of the Lemmy software are @[email protected] and @[email protected].

Each Lemmy instance (there are >600 of them) was started by a different person. This is usually (but not always) the first admin listed in the sidebar on that instance’s homepage. Sometimes the founder is not the most active admin; in many cases they have recruited others to help in order to spread the work and reduce the chance that the instance dies when the founder has some unforeseen life event. Here are a few people who started some of the larger instances:
@[email protected] (mentioned above)
@[email protected]
@[email protected]
@[email protected]
@[email protected]
@[email protected] (took over that instance from a previous owner)